Nope not thinking linearly, I'm thinking layers... you have chosen to remove a layer of security... fair enough.. your choice. You could argue on the same basis that you don't need antivirus, and that you don't set up no passwords, firewall... how far do you wanna go?
BTW it's not just about where the data is situated, you are simply stating here that you have a first line of defense by using another PC.
So how did you configure your LAN and PC? Do you have a DMZ between your PC's via your LAN? I doubt it! and nor do I, I can freely access machines on my local LAN (although there is a firewall between all PC's). Note that malicious software won't necessarily be detected or executed until it is run, so do you run your song on a special install of cakewalk on your test PC to see what happens first and then pass it to your DAW which also has cakewalk installed? Nope I don't do that either. Infected PDF's is a classic example - pass the file around and one day somebody will open the file somewhere if the antivirus has failed to detect. BTW I have no idea if Cakewalk files can take payloads or not, but as files can be archived I would say yes.
Sorry I don't buy the "it's been working for years" or "because I've tested it on another machine it's clean" argument (was valid 20 years ago maybe, when "sheepdip PCs" were widely used and malicious software was in its infancy), nope mostly you just downloaded it and didn't execute it until you found the right machine that runs the right software that will execute it. And if your antivirus hasn't detected anything malicious then a malicious process may run via the application you executed on and it may need admin permissions, which you will be none the wiser for without UAC.
In fact if you want your machine to be entirely safe why not switch it off permanently ! :).
(nb all my machines have internet, I use a decent software and hardware firewall, I have no issues either. But I do know one day I might. Security is never perfect whatever your scenario).